As a display apparatus, a liquid crystal display (LCD) is in widespread use.
The LCD, a display apparatus exploiting liquid crystal, has liquid crystal enclosed in a space between two transparent plates, and displays a picture by applying a voltage thereto for changing the orientation of liquid crystal molecules for thereby changing light transmittance. The liquid crystal, forming an LCD, does not itself emit light, so that the picture is demonstrated using reflected light in a light place, and using the light from plural light emitting units (backlight device) arranged on the rear side, in a dark place. Since the LCD is thinner and lighter in weight than other display apparatus, such as cathode ray tube (CRT) or a plasma display panel (PDP), it is used widely for a mobile computer or a space saving desk top personal computer.
As a light source used in a backlight device, a point light source, radiating light as a dot, such as light emission diode (LED), or a fluorescent tube, radiating light as a line, is used. The backlight device is of an edge light configuration or a subjacent configuration, depending on the disposition of the point or linear light sources.
The edge light configuration backlight device guides the light, radiated from a point light source or a linear light source arranged on a lateral side of the light guide plate, for emitting light by planar light radiation. The subjacent configuration backlight device diffuses light radiated from plural point light sources or linear light sources, arranged directly below the liquid crystal display panel, using a light diffusing plate, to emit light by planar light radiation. The subjacent configuration backlight device includes e.g. a reflecting plate, arranged on the back surface of the light source, for improving the utilization efficiency of light radiated from a point or linear light source for reducing the loss of emitted light.
With the subjacent backlight device, plural point or linear light sources are arranged, at a preset distance from one another, directly below the liquid crystal display panel. Since the respective light sources exhibit light emitting directivity, the light intensity differs depending on the direction of light emission, and hence the light emitted by planar light radiation must be equalized in luminance over the entire surface. For example, if a linear light source is used for the subjacent backlight device, the light intensity is strongest at a location directly above each of plural linear light source, arranged side-by-side with one another, with the light intensity becoming weaker in a direction proceeding away from the light source. Specifically, except if the light is equalized sufficiently, the radiated light forms a replica of the location of the light source, that is, there is displayed a dot-shaped lamp image or a striped dot image for a point light source or a linear light source, respectively.
For example, if the number of the light sources arranged is increased for reducing the spacing between the neighboring light sources, the radiated light may be equalized by raising the degree of light diffusion of the light diffusing plate to a certain extent. However, if the number of the light sources is increased, the cost of the backlight device and hence that of the LCD are raised. On the other hand, if the degree of light diffusion of the light diffusing plate is increased, there is presented a problem that the luminance cannot be increased in proportion to the larger number of the light sources used.
It may be contemplated to increase the separation between the light source and the light diffusing plate to reduce the difference in the distance from the light source up to respective locations on the light diffusing plate. However, such increased distance between the light source and the light diffusing plate may give rise to an increased thickness of the backlight device and hence an increased thickness of the LCD.
Thus, in a subjacent configuration backlight device, employing a fluorescent tube, which is a linear light source, as a light source, a transparent film, having a light dimming pattern, vapor deposited thereon in register with the fluorescent tube, is arranged between the fluorescent tube and the light diffusing plate, or a light dimming pattern is printed on a light diffusing plate. This light dimming pattern is a pattern of dots containing e.g. a light shielding agent in its composition, and shields the light emitted from the fluorescent tube depending on the dot size to provide for equalized luminance.
An example of a backlight device provided with a light diffusing layer and a light volume equalizing layer by a light transmitting dot pattern is disclosed in Japanese Laid-Open Patent publication H06-301034.
In general, the light transmitted through the light diffusing plate generates a lamp image, recognizable by a human eye, unless the difference in luminance between the luminance value at a location directly above a light source and that at a location partway between the light source and the neighboring light source is reduced to 500 cd/m2 or less.
When the light dimming pattern, discussed in connection with the related art is used for equalizing the luminance, the above difference in luminance of 500 cd/m2 or less may be realized with the light dimming pattern which reduces the transmittance for all light rays of the light guide plate to an extremely low value of 50% or less.
However, with the use of this light dimming pattern, the volume of light transmitted through the light diffusing plate is decreased, thus presenting a problem that the luminance is lowered significantly. For eking out this deficiency and providing sufficient luminance for illumination, excess power has to be supplied to the power source, thus wasting the power.